1. Field of the Invention
The present invention relates to a voltage controlled oscillator, and more particularly, to a voltage controlled oscillator that oscillates in a high frequency band.
2. Description of the Related Art
FIG. 5 illustrates a conventional voltage controlled oscillator. The collector of an oscillating transistor 21 is coupled to a ground through a capacitor 22. A feedback capacitor 23 connects the base to the emitter of the oscillating transistor 21, and a capacitor 24 connects the emitter of the oscillating transistor 21 to ground.
A resonant circuit 25 is connected to the base of the oscillating transistor 21. The resonant circuit 25 includes a microstrip 25a, a first and a second varactor diode 25b and 25c. The cathodes of the varactor diodes 25b and 25c are interconnected in series. The anode of the first varactor diode 25b is grounded, and the anode of the second varactor diode 25c is connected to the microstrip 25a through a capacitor 25d. The microstrip 25a is directly connected to ground and indirectly connected to the base of the oscillating transistor 21 through a capacitor 25e. The microstrip 25a is a linear conductor. The length of the microstrip 25a is slightly shorter than a quarter of the wavelength of an oscillation frequency.
The anode of the second varactor diode 25c is grounded through an inductor 26. A control voltage is applied to the cathode of the first varactor diode 25b and the cathode of the second varactor diode 25c through an inductor 27. By varying the control voltage, the oscillation frequency is adjusted.
In the above-described circuit, the internal capacitance of each of the varactor diodes 25b and 25c is controlled by the inductive and capacitive coupling circuit. To oscillate at a high frequency, the capacitance increases, while the Q factor of the resonant circuit decreases. Hence the carrier-to-noise (C/N) ratio of the oscillation signal decreases.
According to a first aspect, a voltage controlled oscillator includes an oscillating transistor. The collector of the oscillating transistor is coupled to ground when the oscillating transistor generates a high frequency output. A microstrip is coupled to the base of the oscillating transistor through a first varactor diode and a capacitor. A second varactor diode couples the microstrip to ground. The length of the microstrip is preferably within about xc2xd to about xc2xe of the oscillation frequency of the voltage controlled oscillator.
Preferably, a cathode of the first varactor diode is coupled to a first end of the microstrip and an anode of the first varactor diode is preferably coupled to ground through a first inductor. Preferably, a cathode of the second varactor diode is coupled to a second end of the microstrip and an anode of the second varactor diode is preferably grounded. A control voltage is preferably supplied to the cathodes of the first and second varactor diodes by a power source or a bias circuit.
Preferably, the microstrip is longer than the above-described conventional microstrip, and hence, the Q factor of a resonant circuit increases. As a result, the C/N ratio of an oscillation signal increases, and the deterioration of the Q factor of the resonant circuit, tuned by the control voltage, does not occur or is minimized. Moreover, the second inductor does not require a large inductance, and hence, the second inductor can be replaced by a second microstrip. Preferably, the second microstrip is shorter than the microstrip coupled between the first and second varactor diodes.